A semiconductor product is produced through several steps including a step of forming a photosensitive resin as an etching resist on a silicon wafer to perform etching for the silicon wafer. This photosensitive resin is liquid-state, so that it is applied onto a surface of the silicon wafer by using a spin coater and the like. However, the photosensitive resin must be dried after applying, and thus the silicon wafer applied with the resin is placed on a heater and heated for drying. Conventionally, a heater used for such a purpose is made of a metal, for example, a heater in which a heating element is arranged on a back face of an aluminum plate has been employed as the heater made of metal.
However, such a heater made of metal has the following problems. First, the thickness of the substrate must be as thick as about 15 mm since the substrate is made of metal. This is because, in a thin metal plate, a bend or a strain is generated because of thermal expansion resulting from heating so that a semiconductor wafer put on the metal plate is damaged or inclined. However, if the thickness of the substrate is made thick, the heater becomes heavy and bulky.
At the time of heating, the heating temperature is controlled by changing the voltage or amperage applied to the heating elements. However, since the metal plate is thick, the temperature of the substrate does not follow the change in the voltage or amperage promptly. Thus, the temperature thereof cannot be easily controlled. Thus, as disclosed in JP Kokoku Hei No. 8-8247, there are suggested techniques of performing the temperature-control of a nitride ceramic with which a resistance heating element is formed, by measuring the temperature in the vicinity of the resistance heating element.
Inventors of the present invention also tried to heat a silicon wafer employing the technique, however at that time, a problem of occurrence of temperature distribution on the surface of the heater took place. Therefore, inventors of the present invention have made enthusiastic investigations of causes of the temperature distribution and consequently, found that a reason for the occurrence of the temperature distribution in spite of temperature control is the fact that: a surface roughness of the ceramic substrate contacting a thermocouple is high and thus the contact of the thermocouple and a temperature measurement element becomes a point contact, so that the heat of the ceramic substrate cannot sufficiently be transmitted to the thermocouple to result in erroneous measurement result by the thermocouple.
Further, also a problem such that the thermocouple attached to the ceramic substrate became easy to be dropped during a long time use took place. The thermocouple is normally fixed on the surface of the ceramic substrate using a solder or an inorganic adhesive material or fixed in the inside of a bottomed hole formed on the bottom face of the ceramic substrate, and if the thermocouple is dropped, a heater becomes uncontrollable, resulting in occurrence of fires and serious incidents.